1. Field of the Invention
The present invention relates to an image forming apparatus using electrophotography such as a copier, a printer, a facsimile, and a complex machine thereof, and specifically, to a density detection technology of detecting density of a developer by density detecting means provided in a transport path of the developer in a developing apparatus using a two-component developer.
2. Description of the Related Art
Conventionally, a two-component image forming apparatus for forming images using a developer containing toner and carrier is arranged so as to form an electrostatic latent image on a photoconductor drum as an image carrier, develop the electrostatic latent image with a developing apparatus, transfer the obtained toner image onto paper by a transfer unit, and fix the image to the paper by a fixing unit. At the same time, an amount of toner in the developing apparatus is always kept constant by sensing the used amount of toner by detecting the toner ratio density (a ratio of an amount of toner in a developer) as developer density with a sensor and supplying toner from a toner bottle into the developing apparatus according to the sensed amount of toner.
As shown in FIG. 14, as a conventional developing apparatus, one provided with a pair of transport mixers 300 and 302 for oppositely transporting a developer along a longitudinal direction, a partition plate 304 that partitions between these pair of transport mixers 300 and 302, a density sensor 306 for detecting the toner ratio density at the time of development on the undersurface of developing apparatus in a development position has been known (e.g., JP-A-2000-122399).
Since the density sensor 306 provides different output voltages depending on variations in bulk of the developer on the density sensor 306 even for the same toner ratio density, a detection error in toner ratio density is caused.
In addition, since a certain amount of developer is required on the density sensor 306 to detect the toner ratio density by the density sensor 306 in the apparatus shown in FIG. 14, a paddle 302b for delaying the transport of the developer is formed in a position facing the density sensor 306 on the transport mixer 302. Further, since the height of the partition plate 304 for partitioning between the transport mixers 300 and 302 is constant, when the toner ratio density in the developer rises and the bulk of the developer increases, the developer is deposited especially on the density sensor 306 by the action of the paddle 302b and the bulk density of the developer on the density sensor 306 becomes higher. The condition is shown in FIG. 15. The rise of toner ratio density and the output of the density sensor 306 are proportional to each other to a certain point (to a value of toner ratio density of about 8.3 wt % in FIG. 15), however, a phenomenon that the output of the density sensor 306 reverses relative to the toner ratio density at the above point when the bulk density becomes extremely high occurs. Accordingly, although the actual toner ratio density is high, the toner ratio density output by the density sensor 306 becomes lower, and thereby, toner is supplied into the developing apparatus more than necessary.
As a technology for solving such a problem, one that prevents rising of the bulk of the developer on the density sensor by reducing the height of the partition plate at the upstream of the density sensor lower than the other part and has a part of the rotational shaft of the transport mixer facing the density sensor made larger in diameter than the other part has been disclosed (e.g., JP-A-2002-148927).
A relationship between the toner ratio density and the sensor output in this case is shown in FIG. 15. Although the toner ratio density at which the sensitivity of the density sensor is reversed is slightly higher than that shown in FIG. 15, a reversal phenomenon also occurs in the sensor output. In the structure, fins are formed on the periphery of the rotational shaft of the transport mixer in order to stir and transport the developer, and the rotational shaft of the transport mixer on the density sensor has a larger diameter than that of the other part and the size of the fins of this part is smaller than that of the other part. Accordingly, the transport speed of the developer on the density sensor becomes extremely slow, and, even when the height position of the partition plate at the upstream of the density sensor is lowered, consequently, a phenomenon that a large amount of developer collects on the density sensor and the bulk density becomes higher occurs. Further, there is another problem that image smudges are produced because the transport path of the developer becomes shorter as the height position of the partition plate at the upstream of the density sensor is lowered than that of the other part, and the developer is transported to the developing sleeve side from the lowered partition plate before fresh toner replenished from the replenishment port is mixed with the developer.